Iwakuma (2-1) missed out on his third win of the season despite striking out eight in six innings, allowing no earned runs and just three hits.
The Angels scored in the sixth off shortstop Robert Andino's error, but were shut down by Iwakuma otherwise. Seattle scored a run each in the seventh and eighth, after Iwakuma left having thrown 92 pitches.
Iwakuma lowered his ERA to 1.67, and has the lowest ERA among all American League starters since last season's All-Star break.
"My fastball was good and I managed to keep everything down," Iwakuma said. "Felix (Hernandez) won for us yesterday so I really wanted to keep it going for us. I thought I pitched the way I'm capable of pitching."
Seattle won three of four from the Angels to take their first series of the year.